This invention relates to an optical memory and in particular to a holographic optical memory.
In the specification, the term "light" is used to mean electromagnetic waves within the band frequencies including infrared, visible and ultraviolet light.
A holographic optical memory makes use of a memory medium upon which many individual holograms are stored. Each hologram represents a different bit pattern or "page". The information is stored by directing two beams to a desired location on the memory medium. One beam, the information beam, contains the bit pattern formed by a page composer, while the second beam acts as the reference beam necessary for holographic storage. To read out the information, a readout beam selectively illuminates one of the holograms stored, thereby producing at a reconstructed image plane a reconstructed image of the bit pattern stored in the hologram. An array of photodetectors is located at the reconstructed image plane to detect the individual bits of the bit pattern.
This type of memory is extremely attractive. In the "bit-by-bit" type of optical memory, a single recorded spot on the memory medium represents only one information bit. On the other hand, a single hologram recorded on the same memory medium represents a page which may contain as many as 10.sup.5 bits. Memories having 10.sup.5 or 10.sup.6 pages have been proposed, with each page containing about 10.sup.5 bits.
Another advantage of the holographic optical memory is that the information stored in the hologram is stored uniformly throughout the hologram rather than in discrete areas. Therefore the hologram is relatively insensitive to blemishes or dust on the memory medium. A small blemish or dust particle on the memory medium cannot obscure a bit of digital data as it can if the bits are stored in a bit-by-bit memory.
One difficulty experienced with certain materials used for memory mediums in holographic optical memories, such as MnBi and certain photochromic materials, is that these materials exhibit a low diffraction efficiency. Therefore the signal received by the photodetector array is rather low. As a result the signal-to-noise ratio during the readout stage is also low. Although the intensity of the light received by the photodetector array can be increased to some extent by increasing the power of the readout beam, the readout beam power must not be so great that the information is erased or the film destroyed.